1. Field of the Invention
The present invention relates to communications systems and, more specifically, to an apparatus and method for determining BTSC filter coefficients.
2. Description of the Prior Art
BTSC-compatible encoders are used to encode stereo audio signals into television signals. Existing systems typically use an analog approach. The analog approach is environmentally sensitive and requires extra circuitry to be compatible with digital television systems.
Existing digital BTSC require complex circuitry to facilitate determination of BTSC filter coefficients. Such circuitry is made complex because the feedback functions used to calculate the coefficients tend to exhibit a sharp bend, thereby causing the function to behave in a nonlinear fashion. Such non-linearity reduces the precision of interpolated coefficients.
Therefore, there is a need for a circuit that employs a nearly linear aspect of the BTSC feedback function to facilitate more precise interpolation of BTSC filter coefficients.
The disadvantages of the prior art are overcome by the present invention, which in one aspect is a method of calculating a variable spectral compression filter coefficient in a BTSC compatible stereo encoder from a feedback variable. A reciprocal value that is proportional to a reciprocal of the feedback variable is calculated. A lookup table is indexed using a preselected set of bits of the reciprocal value to generate at least one parameter of the filter coefficient function. An approximation of the compression filter coefficient is linearly interpolated based on the at least one parameter of the filter coefficient function.
In another aspect, the invention is a method of calculating a variable spectral compression filter coefficient in a BTSC compatible stereo encoder from a feedback variable. A reciprocal value that is proportional to a reciprocal of the feedback variable is calculated. A lookup table is indexed using a preselected set of high order bits of the reciprocal value. A first portion of the lookup table stores a plurality of discrete values at index points of a line segment corresponding to a filter coefficient function approximation. The first portion of the lookup table generates an initial discrete value corresponding to the filter coefficient function at a value of the high order bits. A second portion of the lookup table stores a plurality of slope values with each slope value indicating a slope of a line segment of the filter coefficient function. The second portion of the lookup table generates a slope value of a line segment corresponding to the filter coefficient function at the value of the high order bits. An approximation of the compression filter coefficient is linearly interpolated based on the slope value, the initial discrete value and a preselected set of low order bits of the reciprocal value.
In yet another aspect, the invention is an apparatus that includes circuitry that calculates a variable spectral compression filter coefficient in a BTSC compatible stereo encoder from a feedback variable. The apparatus includes a reciprocal value circuit that calculates a reciprocal value that is proportional to a reciprocal value of the feedback variable. The apparatus also includes a lookup table that is indexed by a preselected set of high order bits of the reciprocal value, wherein a first portion of the lookup table stores a plurality of discrete values at index points of a line segment corresponding to a filter coefficient function approximation, so as to generate an initial discrete value corresponding to the filter coefficient function at a value of the high order bits, and wherein a second portion of the lookup table stores a plurality of slope values, each slope value indicating a slope of a line segment of the filter coefficient function, so as to generate a slope value of a line segment corresponding to the filter coefficient function at the value of the high order bits. A linear interpolation circuit interpolates an approximation of the compression filter coefficient based on the slope value, the initial discrete value and a preselected set of low order bits of the reciprocal value.